1. Field of the Invention
The present invention relates to an electronic musical instrument of the type that generates a musical waveform by computing its amplitude value at each sample point through Fourier synthesis, and more particularly to an electronic musical instrument which is adapted so that two kinds of musical waveforms calculated and synthesized one after the other are interpolated at shorter time intervals, thereby effectively obtaining temporal variations of the musical waveforms with a small volume of waveform synthesis calculation.
2. Description of the Prior Art
Heretofore, there have been proposed many digital type electronic musical instruments which produce the amplitude value of a musical waveform at each sample point thereof by some methods and read it out at a readout rate corresponding to the note frequency of a musical sound to be produced. The simplest one of them is what is called a "waveform-memory method" which stores and reads out waveform data itself, and a method that converts an analog input to digital form to obtain waveform data is also one of the simplest methods. However, these conventional methods are defective in that an enormous memory capacity is needed for varying the musical waveform with the note range of a musical sound to be produced and in that the musical aveform undergoes no temporal variations. Furthermore, there have also been proposed a method of computing parameters through the use of various continous functions and a method of computing temporal variations of the musical waveform in a real-time waveform synthesis by a frequency modulation method, but the correspondence between a parameter for the waveform generation and the timbre of the musical sound actually produced is unnatural to the human sense, and a desired timbre is difficult to obtain.
On the other hand, a musical waveform generating system utilizing Fourier synthesis has undergone various improvements for making up for the defect of a large volume of waveform synthesis calculation and has been widely employed since parameters for harmonic coefficients naturally correspond to an auditory evaluation of timbre. In the musical waveform generation system utilizing Fourier synthesis, it is the component ratio of a harmonic coefficient that determines the timbre of a musical sound. As a method for causing temporal variations in the musical waveform, there has been suggested a method of selecting many harmonic coefficients by using a plurality of memories, but this method has such a shortcoming that sufficient timbre variations cannot be obtained in spite of an enormous circuit scale. Furthermore, there have been proposed a system which multiplies a preset harmonic coefficient and a parameter of a Formant filter, as described in Japanese Patent Publication No. 46445/78, and a system which multiplies a temporal variation function for each harmonic coefficient, as described in Japanese Patent Public Disclosure No. 172396/84. With these conventional systems, however, a harmonic coefficient multiplying circuit is required, and in addition, since the calculation of the musical waveform involves a multiplication and an accumulation, calculations of temporal variations of each harmonic coefficient of higher order are limited in terms of the scale of circuit, the volume of calculation and the calculating speed, the temporal variations calculated are not satisfactory as temporal variations in the digital type electronic musical instrument.